The present invention relates to a method and apparatus for measuring the thicknesses of various materials efficiently and precisely.
At present, it is a laser thickness measuring machine for block gauge calibration use that is traceable and capable of measuring material thicknesses with the highest precision. Because of its application specified for block gauge measurements, however, the laser thickness measuring machine has a limitation on the size of the specimen to be measured. Further, the laser thickness measuring machine calls for sufficient knowledge and much skill, in particular, for precise measurement of the thickness of a transparent material like glass or a silicon (Si) single crystal with oxidized surface, since a phase shift occurs when the laser light is reflected from such a measuring object or specimen. Additionally, the laser thickness measuring machine is expensive.
For the reasons given above, a contact measuring method is widely used to measure the thicknesses of various materials through utilization of a linear gauge or the like. A conventional measuring system using such a method comprises, as depicted in FIG. 1, a measuring element 1, a thickness surface plate 2, and, if necessary, an auxiliary jig 4 that is used to provide increased repeatability or accuracy of measurement. The measuring system is placed on a vibration isolation table, and the measurement is conducted in a constant temperature room. The measuring temperature is usually 20xc2x11xc2x0 C., preferably, 20xc2x10.5xc2x0 C. for measurement with higher accuracy. An ordinary measurement procedure begins with bringing the measuring element 1 into contact with the surface of the surface plate 2 as depicted in FIG. 1A (or the auxiliary jig 4 in FIG. 1C) to define a reference point for measurement. The next step is to place a measuring object or specimen 3 on the surface plate 2 as depicted in FIG. 1B (or the auxiliary jig 4 in FIG. 1C), followed by bringing the measuring element 1 into contact with the specimen 3. Disposed between the surface plate 2 and the specimen 3 in FIG. 1C is the auxiliary jig 4 that has a vacuum-suction capability and hence ensures fixing the specimen 3 for stable measurement. In this instance, the thickness of the specimen 3 is measured as the distance from the reference point to the tip of the measuring element 1.
In many cases, the accuracy of the linear gauge in the thickness measuring machine using the above-described method is used intact as the accuracy of the thickness measurement.
In the actual measuring system, however, the accuracy of measurement is influenced, for example, by the rigidity of a thickness gauge stand, or the flatness of the surface plate 2, the flatness and parallelism of the auxiliary jig 4 and the specimen 3, and a warp in the specimen 3. The specimen 3 may sometimes be measured while left curved as depicted in FIG. 2. The measuring accuracy is affected as well by distortion of the specimen 3 that is caused by its contact with the measuring element 1 or auxiliary jig 4. This incurs the possibility that the difference between the measured value and the true thickness of the specimen 3 is fairly larger than the accuracy guaranteed by the measuring system.
Further, a stable measuring environment is indispensable for accurate thickness measurement, but occasionally the prior art does not give due consideration in this respect. In particular, the stability of the measuring temperature, the absence or presence of vibration, and the cleanness of the measuring room greatly affect the accuracy of measurement, and hence much attention should be paid to them. The measurement is usually carried out in a constant temperature room, and the stable point of room temperature varies with the numbers of people and in-service devices in the room, and the local temperature in the room fluctuates with comings and goings of people and ON/OFF operations of devices installed in the room. On this account, the temperature stable point during measurement, which affects the accuracy of measurement, undergoes about xc2x11xc2x0 C. variations with measurement conditions, allowing temperature fluctuations during measurement. For example, in the case of measuring a block gauge of a 10 mm nominal size [Literature 1] with a xc2x10.5xc2x0 C. temperature stability, its linear expansion coefficient of approximately xc2x110xe2x88x925 Kxe2x88x921 may sometimes cause a measurement error of around xc2x10.05 xcexcm.
As described above, the prior art has many problems that should be taken into account for contact thickness measurement with xe2x80x9chigh accuracy,xe2x80x9d and hence it does not allow ease in measuring accurately to xc2x10.1 xcexcm.
It is therefore an object of the present invention to provide a method and device for measuring absolute values of the thicknesses of various materials efficiently and precisely.
The thickness measuring apparatus according to the present invention comprises:
a constant-temperature chamber;
a vibration isolation table placed in said constant-temperature chamber;
a linear gauge provided with a surface plate mounted on said vibration isolation table and a measuring element for contacting a specimen under measurement mounted on said surface plate from above, for outputting information about the position of said measuring element; and
a specimen table mounted on said surface plate and having a centrally-disposed circular protrusion for receiving said specimen, said centrally-disposed circular protrusion having a flat surface of a diameter sufficiently smaller than said specimen.
The thickness measuring method according to the present invention comprises the steps of:
(a) bringing down said measuring element of said linear gauge from above into contact with a centrally-disposed circular protrusion of a specimen table mounted on said surface plate to measure the position of said measuring element as a first position, said centrally-disposed circular protrusion having a diameter sufficiently smaller than said specimen;
(b) mounting said specimen on said centrally-disposed circular protrusion and bringing down said measuring element of said linear gauge from above into contact with said specimen to measure the position of said measuring element as a second position; and
(c) calculating the thickness of said specimen from the first and second positions.